Elaborate signifies here a complexity or extensiveness of design. The house is larger and perhaps more architecturally involved (or interesting) than was expected.

"Cheerful" reflects the positive impression that the house makes on the narrator, Nick.

Taken together, this passage suggests that Nick's hopes for the evening are intact when he arrives at the house. The outward trappings of life here in the east is clearly appealing to Nick (or at least to his initial mindset).

It's worth remembering that Nick comes east with hope, like Gatsby, and expects to grow as a person - to attain some sophistication. On his first visit to Daisy's house, this is what Nick is seeking and, at first blush, this is what he finds.

Quickly, however, Nick discovers that the outwardly pleasant show is the costume on an inwardly tawdry drama.